The invention relates to a method and an apparatus for producing a rim, and a rim produced using said method and provided in particular for bicycles and the like.
In the field of amateurs and in professional cycling, the stress tolerance and the weight of the components are a decisive factor since the users set great store by high-quality, lightweight components. One aspect of importance includes in particular the component quality.
To reduce the weight of bicycle rims, rims of fiber-reinforced plastics have become known for which while their total weight is reduced the stability achieved is comparable to that of metal rims.
Thus, for example EP 1 231 077 B1 has disclosed a method for producing a rim for a bicycle and an apparatus for producing a rim according to which, to produce a hollow rim, a specific quantity of layers of a structural woven fiber which includes a plastic matrix, are successively, one by one applied on the inner part of a mold. The applied, predetermined quantity of layers forms the inner wall, the outer wall, the two side walls, and the load-bearing areas of the rim. Subsequently an inflatable bag is applied on the layers. The inflatable bag is inflated slightly to assume its provided volume. Subsequently a quantity of layers namely, a first predetermined quantity of layers is folded back directly on the slightly inflated bag and folded on said bag such that the bag is entirely, on all sides enclosed by said first, predetermined quantity of layers. Subsequently a core is applied on the folded back layers. The core has a high coefficient of thermal expansion for exerting pressure on the layers at the increased curing temperature. Then the remaining layers, i.e. a second predetermined quantity of layers is folded directly on the core. Then the mold is closed by means of an outer part and the slightly inflated bag is inflated with air to its full operating pressure to press the layers against the mold. Then the mold is heated up to an increased temperature which is sufficient to generate reticulation of the plastic matrix. The bicycle rim is demolded and the core is then removed. The protruding rim flanges are trimmed and in a subsequent mechanical machining step given the desired shape.
A drawback of the known method is that the layers are placed in the mold one by one and thus the reproducibility of the production method is limited. Another drawback is that the particularly stress-loaded points in the rim base where as a rule the spokes are attached, the material thickness is low while in other places it is larger than necessary. The known rims provide for a uniform wall thickness over the whole area of the rim base.
Therefore it is the object of the present invention to provide a different method for producing a bicycle rim which allows to provide a higher reproducibility and in which the rim base can have various and in particular increased wall thicknesses.